This has literally only happened to me once. After catching a Pikachu in Viridian Forest, I didn't replace the Balls I'd used to catch it, and then BAM, Shiny Metapod. I wouldn't mind, but Metapod has a reasonably easy catch-rate, so I wouldn't have needed to do much, had I the Pokéballs... :/

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This one's called 'Blue', and it's very Pokémon themed, so have a listen! Check out the links below for more stuff, if you like this!